Study

The influence of shade and root competition on restablishment of bluebunch wheatgrass Pseudoroegneria spicata in a big sagebrush Artemisia tridentata-dominated site near Logan, Utah, USA

  • Published source details Huber-Sannwald E. & Pyke D.A. (2005) Establishing native grasses in a Big Sagebrush–dominated site: an intermediate restoration step. Restoration Ecology, 13, 292-301

Summary

In the Great Basin region, USA, big sagebrush Artemisia tridentata and perennial tussock grasses, such as bluebunch wheatgrass Pseudoroegneria spicata, co-dominated undisturbed sagebrush steppe communities prior to European settlement. Many of these semi-arid plant communities are shifting to dominance by woody species as a consequence of land degradation through intense cattle grazing and fire suppression. Heavy grazing eventually leads to a reduction in grasses, which results in the removal of the main source of continuous fuel (i.e. dead grass stems and leaves) for fires. When combined with intentional fire suppression, overgrazing results in increased dominance of fire sensitive species mostly unpalatable to cattle, such as big sagebush.

Rehabilitation efforts in sagebrush steppes have tended to focus on shrub removal and introduction of forage grasses to successfully revert from shrubland to grassland. For conservation purposes, native species should be included in restoration projects and retention of some woody plants is desirable. In this study, the potential for interseeding grasses into dense shrub communities as a precursor to thinning shrubswas examined. Seedling establishment of the native bluebunch wheatgrass was compared with that of the Eurasian crested wheatgrass Agropyron desertorum in dense big sagebrush stands.

Study site: This field experiment was conducted over a 20-month period in 5-year-old big sagebush Artemisia tridentate experimental plots near Logan, Utah, southwestern USA. The sagebrush plants were 60-70 cm tall and approximately 50 cm apart.

Grass seedling planting: On the north and south sides of the sagebrushes, eight grass seedling populations were planted in one of a combination of four shade levels (0, 40, 70, and 90% reduction of solar radiation) and initial root-exclusion treatments (present/absent) on the establishment and growth of P.spicata and A.desertorum seedlings.

The interference effects of sagebush on the two grasses were assessed and the best microsites for grass establishment in the sagebrush–dominated communities identified.

Grass survival & growth: The contrasting light conditions (no shade to 90% shade) in the dense big sagebrush communities did not affect the final survival of P.spicata and A.desertorum seedlings after the 20 months of the study. However, initially A.desertorum emerged as seedlings much earlier in spring (March) 1995 than did P.spicata and most of these died early due to frost heaving and subsequent desiccation of their exposed roots. There was no treatment affect on the early mortality of A.desertorum seedlings.

Fifty to 85% of the P.spicata and seedlings survived the first summer after planting through to the summer of 1996. Neither shading nor root exclusion from A.tridentata affected final seedling survival of either species. However, seedling biomass of both grasses was reduced by initial root interactions with A.tridentata. Despite this, in all shade and root-exclusion treatments, some seedlings of both species developed into large individuals.

Conclusions: Most vegetation restoration techniques for semiarid Artemesia rangelands involves at least some removal of woody plants before reintroducing of grasses and herbs. These results indicate that reintroduction before reducing the abundance of the woody plants may be feasible. No marked differences were detected in establishment, survival, or growth between the two grass species trialed and the native P.spicata appears suitable for restoration purposes. Results also indicate that when using P.spicata seeds for restoration purposes in dense sagebrush stands, that light conditions should not affect seedling establishment and growth unless light is greatly reduced (>70%). Thus, seeding on south sides of sagebrush stands is recommended to overcome this potential problem. Root interaction with sagebrush decreased average seedling biomass, thus preferred seeding or transplant areas should be in the transition of undershrub canopies and in gaps to minimize this effect. Thus, the use of this interseeding technique shows promise for restoring grasses within dense A.tridentata stands and should be given consideration when shrub retention is important.


Note: If using or referring to this published study, please read and quote the original paper. Please do not quote as a conservationevidence.com case as this is for previously unpublished work only. The original paper can be viewed at: http://www.blackwell-synergy.com/journal.asp?ref=1061-2971

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